Gas/fluid mass exchange apparatuses are commonly used in medical practice for transferring oxygen from air to a blood supply and carbon dioxide from blood to the air. Such devices are typically referred to as oxygenators and most frequently employ pure oxygen as the gas phase. However, none of the designs build on the fundamental science that determines the most effective arrangement of components. The basic principles that must be observed are:                1. Minimize the risk of blood clots forming within or being stimulated by the flow through the mass exchanger;        2. Maximize the mass transfer rates of oxygen and carbon dioxide within the exchanger; and,        3. Minimize the volume of blood in the exchanger (that is the volume of blood outside the body).        4. Minimize pressure drop across the exchanger.        
The risk of blood clots can be minimized by applying a treatment to the surface of all materials contacting blood. The surface can be either non-thrombogenic or anti-thrombogenic, or can combine non-thrombogenic and anti-thrombogenic properties. However, no surface completely eliminates the risk of clots forming when the blood flows outside the veins. The rheology of blood changes whilst it passes through any medical device and it clots at a rate determined by the characteristics of the blood and of the surfaces with which it is in contact.
In order to maximize mass transfer rates, the blood flow characteristics must be understood. In particular, any design must recognize that the majority of the blood flow will be laminar. The laminar characteristics of the flow are made clear by reference to the relevant Reynolds Number, namely:Re=ρud/μwhere                Re is the Reynolds Number        ρ is the fluid density        u is the fluid velocity        d is a characteristic linear dimension (for example, the diameter of a tube)        μ is the fluid viscosity        
Blood is a non-Newtonian fluid, but for the purpose of estimating Reynolds Number an apparent viscosity can be taken. In this respect, at a temperature of approximately 37° C., the value of the Reynolds Number typically resides in the range 0.06<Re<12.
The turbulent flow transition occurs at a Reynolds Number of around 2,000. Hence, within a large margin, the flow is substantially laminar. The non-Newtonian nature of blood introduces uncertainty, but the margin to laminar flow is so great that it can be assured that flow remains substantially laminar. Under laminar conditions, mass transfer is essentially by diffusion, which is similar to heat transfer by conduction, and for geometrically similar flow patterns, both heat transfer and mass transfer coefficients are found to be inversely proportional to a characteristic linear dimension. For flow through tubes of circular cross-section, Coulson & Richardson (“Chemical Engineering”, Volume 1, 6th Edition, p 425, equation (9.80)) derive the relationship for heat transfer as:h=4.1k/d where “h” is the heat transfer coefficient, “k” is the thermal conductivity, and “d” is the inner diameter of the tube. The corresponding equation for mass transfer is:U=4.1D/d where “U” is the mass transfer coefficient, and “D” is the diffusivity of the material being transferred within the bulk fluid through which it is transferred. Similar equations apply for other geometries, but with different values of the numerical coefficient.
In gas/fluid or more particularly, gas/blood mass exchange, the linear dimensions are very small compared to the length with typical length/diameter ratios of 50 to 200. Hence, the end and exit effects are small, and no correction needs to be made for the effect. In practice, blood is non-Newtonian and the flow patterns may be more complex. However, it still follows that mass transfer coefficients are almost independent of Reynolds Number and hence of fluid velocity. It further follows that mass transfer rates depend primarily on interfacial area, driving force (difference between gas phase pressure and equilibrium partial pressure), and the mean width of the flow channels.
U.S. Pat. No. 6,004,511 discloses a gas/fluid mass exchanger in which a blood supply is passed from a patient, through a flow region comprising a plurality of closely packed gas permeable flow ducts through which air arranged to pass. The oxygenated blood with depleted carbon dioxide is then returned to the patient. The diffusivity of gas to and from blood across the flow ducts is known to depend on the viscosity of the blood, such that for example as blood clots, its apparent viscosity increases and the diffusivities of oxygen and carbon dioxide would be expected to decrease. Clot growth is initiated at the surfaces over which blood flows and so a slow blood flow at a surface presents two major problems. These include the intrinsic risk that clots present to the well-being of a person and that the presence of clots increases the effective viscosity of the blood, so that molecular movement is hindered and the diffusivities are reduced.
The blood clotting process progresses when blood leaves the blood vessels and travels over a foreign surface, such as a mass exchanger surface. The longer the blood is out of the blood vessels, the greater the risk and extent of blood clotting. It follows that the longer the residence time of blood out of the blood vessels, the greater is the risk that harmful clots will be returned to the body, with both detrimental impact on a patient's health and risk that a clot will cause death. As part of the clotting process, the apparent viscosity of the blood increases, and the longer the blood is out of the body, the further this thickening process proceeds. This thickening has a detrimental impact on mass exchanger performance because increasing viscosity decreases the diffusivities of oxygen and carbon dioxide in the blood. Reduced diffusivity results in reduced mass transfer and reduced exchanger performance. There are therefore strong incentives to minimize blood residence time in such a mass exchanger.
U.S. Pat. No. 6,004,511 discloses a gas/fluid mass exchanger comprising a plurality of hollow fibres which are packed closely together in touching relation, to maximize the total surface fibre area per unit volume. The effect of close-packed fibres is illustrated in FIG. 1 of the drawings, which presents a sectional view across a known mass exchanger 10. In this illustration, air is fed through narrow tubes 11 and blood is passed around them. The regions 12 (illustrated as shaded) adjacent to where the fibres touch 13 give rise to slow blood flows. Blood clots 14 can readily develop in regions where the blood flow becomes stagnant or near-stagnant. In order to transfer from the free flowing stream to the membrane surfaces, the dissolved gas must diffuse through these stagnant or clotted areas. The long transfer paths almost tangential to the fibre surface, result in low mass transfers coefficients. Thus, these shaded regions effectively block mass transfer giving rise to reduced effective mass transfer area.
The hollow fibres and membranes typically used in mass exchange apparatuses are necessarily thin to enable the gas molecules to pass therethrough. However, the thin form of the fibres makes for a flexible fibre which can move and touch neighbouring fibres, under the fluid flow and thus trap blood in the interstices, which can lead to the above mentioned problems. This problem is overcome in some commercial applications by supplying fibres wound into a mat comprising supporting fibres which maintain a defined separation between the hollow fibres. These include for example, the Celgard X30-24 and the Oxyplus 90/200 hollow fibres. Such mats have the disadvantage that the supporting fibres making up the mats are nearly normal to the blood flow and produce local areas of virtually zero flow at each connection, thus giving both long residence times and a high surface area with no mass transfer, which present opportunities for blood clot growth.